1. Field of the Invention
This invention relates to a yellow emitting phosphor and a low-velocity electron excited fluorescent display device utilizing the same, and more particularly relates to a yellow emitting zinc oxide-based phosphor and a low-velocity electron excited fluorescent display device containing the phosphor as a fluorescent screen.
2. Description of the Prior Art
As a phosphor which emits light of high luminance under excitation of low-velocity electrons, a zinc oxide phosphor (ZnO) has been commonly known. The ZnO phosphor can be prepared by firing ZnO raw powder alone in a reducing atmosphere, or by firing in the air ZnO raw powder contaminated with an extremely small amount of zinc compound other than ZnO such as zinc sulfide (ZnS) or the like, and the ZnO phosphor emits green light of high luminance when excited by low-velocity electrons. The low-velocity electron excited fluorescent display devices having the fluorescent screen comprising the aforesaid ZnO phosphor have been practically used as display devices for, e.g., desk top and portable electronic calculators and various kinds of measuring instruments.
As is well known, a low-velocity electron excited fluorescent display device (hereinafter referred to as "fluorescent display device") in general has a fundamental structure such that both an anodic plate having a fluorescent screen on one side thereof and a cathode standing face to face with the above-described fluorescent screen are enclosed in an evacuated tube. The fluorescent screen applied on the anodic plate emits light when excited by low-velocity electrons emitted from the cathode. Both FIGS. 1 and 2 schematically show the typical structures of fluorescent display devices, and they show a diode type display tube and a triode type display tube, respectively. As shown in both FIGS. 1 and 2, one side of an anodic plate 11 made of, for example, an aluminum plate, has a fluorescent screen 12 thereon. The other side of the anodic plate 11 is supported by a ceramic base plate 13. The diode type display tube is equipped with a cathode standing face to face with the above-described fluorescent screen 12 placed on the anodic plate 11, and the fluorescent screen 12 is excited by low-velocity electrons emitted from the cathode 14 and emits light. In particular, the triode type display tube shown in FIG. 2 additionally has a grid electrode 15 between the cathode 14 and the fluorescent screen 12 so as to control or diverge the low-velocity electrons emitted from the cathode 14. Moreover, when the surface of the fluorescent screen 12 has a wide area, two or more cathodes may be additionally provided in both fluorescent display tubes shown in FIGS. 1 and 2 wherein only one cathode is provided, and there is no particular limit to the number of cathodes that can be provided therein. The aforesaid anodic plate 11 having the fluorescent screen 12 on one side thereof, the ceramic base plate 13 and the cathode 14 (which are shown in FIG. 1), or the aforesaid anodic plate 11 having the fluorescent screen 12 on one side thereof, the ceramic base plate 13, the cathode 14 and the grid electrode 15 (which are shown in FIG. 2) are enclosed in a transparent container 16 made of, for example, glass, the pressure within which is held at a high vacuum of 10.sup.-5 to 10.sup.-9 Torr.
Although, as is described above, the green emitting ZnO phosphor has been commonly known as a phosphor which emits light of high luminance under excitation of low-velocity electrons, and the fluorescent display device having the above-mentioned fundamental structure and having the fluorescent screen comprising the ZnO phosphor has been practically used as a green emitting display device, there is an increasing demand for phosphors which can emit light of high luminance other than green light under excitation of low-velocity electrons in order to meet the increasing practical use of fluorescent display devices.
As phosphors which can emit light of high luminance other than green light under excitation of low-velocity electrons, red or blue emitting fluorescent compositions comprising a mechanical mixture of indium oxide (In.sub.2 O.sub.3) or zinc oxide (ZnO) and a red or blue emitting phosphor have heretofore been known (Japanese Patent Publication Nos. 23916/1977 and 23911/1977, and Japanese Patent Application Laid Open Nos. 145479/1976 and 115787/1977). In these fluorescent compositions, the emission efficiency of the phosphor under excitation of low-velocity electrons is markedly elevated by the conductive material mixed therewith. However, differently from the fluorescent compositions as described above a phosphor which can solely emit light of high luminance other than green light under excitation of low-velocity electrons has heretofore hardly been known.